3.24 Exothermic Reactions
FIGURE 3.36
Note: ∆H represents the change in en-
ergy.If the energy produced in an exothermic reaction is released as heat, it results in a rise in temperature. As a result,
the products are likely to be warmer than the reactants. That’s what happens in the exothermic reaction at the URL
below.
http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/chemreac/energychangesrev1.shtml
Q:You turn on the hot water faucet, and hot water pours out. How does the water get hot? Do you think that an
exothermic reaction might be involved?
A:A hot water heater increases the temperature of water in most homes. Many hot water heaters burn a fuel such as
natural gas. The burning fuel causes the water to get hot because combustion is an exothermic reaction.
Combustion as an Exothermic Reaction
All combustion reactions are exothermic reactions. During a combustion reaction, a substance burns as it combines
with oxygen. When substances burn, they usually give off energy as heat and light. Look at the big bonfire in the
Figure3.37. The combustion of wood is an exothermic reaction that releases a lot of energy as heat and light. You
can see the light energy the fire is giving off. If you were standing near the fire, you would also feel its heat.
Summary
- An exothermic reaction is a chemical reaction in which less energy is needed to break bonds in the reactants
than is released when new bonds form in the products. - During an exothermic reaction, energy is constantly given off, often in the form of heat.
- All combustion reactions are exothermic reactions. During combustion, a substance burns as it combines with
oxygen, releasing energy in the form of heat and light.